1. Field of the Invention
This invention relates to an electric parking brake system for a vehicle adapted to automatically control the application and release of a parking brake in accordance with the operating conditions of the vehicle.
2. Description of the Prior Art
In recent years, there have been proposed, in place of the conventional manual parking brake systems, various kinds of electric parking brake systems for vehicle use which include an electromagnetic means for actuating and de-actuating the parking brake system, a group of sensors for detecting the operating conditions of the vehicle, and a controller adapted to receive signals from the sensors for sending out a brake-application command signal or a brake-release command signal to the electromagnetic means so that application and release of the parking brake is controlled in an automatic manner in accordance with the operating conditions of the vehicle. For example, Japanese Unexamined Utility Model Publication No. 48(1973)-21329 discloses an electric parking brake system for a vehicle in which a parking brake is automatically actuated by means of a signal indicative of the depression of a foot brake and a signal indicative of the stopped condition of the vehicle, and de-actuated by means of a signal indicative of the depression of an accelerator pedal. Also, in the Japanese Unexamined Utility Model Publication No. 54(1979)-105429, there is described an electric parking brake system in which a parking brake is automatically actuated by means of a foot-brake depression signal and a push-button operation signal, and de-actuated by means of an accelerator-pedal depression signal or a clutch-pedal depression signal. Further, Japanese Unexamined Patent Publication No. 55(1980)-140634 teaches that a parking brake can be automatically released by means of an accelerator-pedal depression signal and a signal indicative of the shift-gear position of a transmission.
With such an automatically-controlled electric parking brake system, it is desirable from the point of view of safety to provide a manual switch which is adapted to be turned on or off, whenever necessary, by the vehicle operator for the purpose switching the parking brake from its automatic mode of operation to its manual mode of operation.
In this case, if the electromagnetic means for controlling the actuation and de-actuation of the parking brake is supplied with electricity from a battery, the parking brake will be released if the manual switch should be thrown into operation. This would be particularly dangerous if done, for example, by a child in the absence of the operator when the vehicle is parked on a slope with the engine stopped and the parking brake applied since it would free the vehicle to roll down the slope out of control.
Further, when a vehicle is driven in reverse, the driver will frequently stop the vehicle for the purpose of ensuring safety so that the parking brake will repeatedly applied and released. As a result, the backward movement of the vehicle is frequently interrupted by repeated stoppages and hence becomes jerky, thus making it impossible to back the vehicle up smoothly us, for example when backing into a garage.
Also, the parking brake is automatically released by depression of the accelerator pedal upon starting of the vehicle with the automatic control mode selected. Therefore, in order to release the parking brake in an automatic fashion during forward starting of the vehicle on a steep downhill grade or during backward starting on a steep uphill grade, it is required to depress the accelerator pedal. But, in this case, the depression of the accelerator pedal causes the vehicle to start rapidly, thus resulting in a dangerous situation from the point of safety.
Moreover, it would be very dangerous if the manual brake switch should be erroneously operated by the operator or other person in the vehicle to actuate the parking brake when the vehicle is travelling at a relatively high speed.
In addition, should the foot brake be abruptly depressed to brake the vehicle in an urgent manner, the wheels of the vehicle would be locked so that the speed signal from the locked wheels, representative of the travelling speed of the vehicle, would become nearly zero, wherein the vehicle is regarded as being stopped. As a result, the locked state of the wheels is maintained owing to the continued application of the parking brake, thus endangering further travel of the vehicle.